Photolithography is a process that is used to create small structures, such as integrated circuits and micromachines. The photolithography process entails exposing a radiation-sensitive substance, called photoresist, to a pattern of light or other radiation. The pattern is typically created by passing the radiation through a mask, which is composed of a substrate with a pattern on its surface. The pattern blocks some of the radiation or changes its phase to create exposed and unexposed areas on the radiation-sensitive material. In a binary intensity mask, the pattern is made of a light absorbing material on an otherwise transparent substrate. In a phase shift mask, the pattern consists of material that shifts the phase of the light passing through it to create an interference pattern on the photoresist that produces a sharp image. The image produced on the photoresist is referred to as the “aerial image” of the mask. The size of the structure that can be produced is limited by the wavelength of radiation used; shorter wavelengths can produce smaller structures.
As photolithography processes are called upon to produce ever-smaller structures, lithography systems are being developed that use smaller wavelengths of radiation, including ultraviolet and x-ray radiation. (The terms “light” and “photolithography” are used in a general sense to also include radiation other than visible light.) Systems are now being developed that can produce structures having dimensions 70 nm and smaller. Such structures can be fabricated by photolithography using light having a wavelength of 193 nm or 157 nm. Some photolithography masks used with such short wavelengths use reflective, rather than a transmissive, patterns on the masks because the substrates are not sufficiently transparent to such small wavelengths of radiation. In such masks, radiation is reflected from the mask onto the photoresist.
A photolithography masks must be free of manufacturing imperfections if the mask is to accurately produce the desired exposure patterns. Most newly fabricated masks have some defects such as missing or excess pattern material and, before such masks can be used, the defects must be repaired. The requirement for smaller wafer features in photolithography places increasing demands upon the techniques used to repair defects on the photomasks.
In the case of an opaque defect, that is, the presence of extra absorb or phase shift material, the defect can be repaired by removing the extra material using charged particle beam, for example, a focused beam of gallium ions. Unfortunately, the ion beam also damages the mask surface and implants ions into the substrate, which adversely affects the transmission of light through the substrate. As shorter wavelengths are used in photolithography, imperfections in the substrate have a greater effect on the aerial image of the mask. Any alteration of the substrate caused by the repair affects the mask performance, so new mask repair techniques are needed that will reduce the effect of the repair on the substrate.
One method of reducing the effects of ion beam mask repair entails scanning a focused ion beam across a repaired area in the presence of an etchant gas, such as xenon-difluoride, to remove a surface layer of a substrate to improve light transmission. Such a process is described, for example, in U.S. Pat. No. 6,335,129 to Asano et al. U.S. Pat. No. 6,042,738 to Casey, Jr. et al. similarly describes scanning a charged particle beam across a repaired area in the presence of xenon-difluoride. Removing substrate material, however, changes the thickness of the substrate, which in turn changes the phase of the transmitted light. The altered phase alters the aerial image, adversely affecting mask performance, particularly in a phase shift mask. Also, there are limits to how much implanted gallium can be removed by a process that entails directing a gallium beam toward the surface. Thus, a method of restoring a mask close to its original performance is needed.